The present invention relates to a temperature sensing circuit and a temperature sensing method for a voltage drive type semiconductor device.
As one of the protective functions typically provided for a conventional IGBT (Insulated Gate Bipolar Transistor) module containing IGBTs, representing a kind of voltage drive type semiconductor device, temperature sensing using a temperature sensing circuit is employed for detecting a temperature rise of the IGBT module in order to switch off the IGBT before it becomes damaged. For example, such a conventional IGBT module is disclosed in Japanese Patent Application laid-open No. 7-67389.
The temperature rise of an IGBT module is detected by a temperature sensing circuit using a thermistor, and the signal produced by the temperature sensing circuit is transmitted to a control circuit to control the operation of the IGBT. The thermistor is attached on a cooling fin on which the IGBT module is mounted; and so, when the temperature of the cooling fin rises, the resistance of the thermistor is decreased and the terminal voltage thereof is also decreased. When the temperature exceeds a preset temperature, the temperature sensing circuit operates to switch off the IGBT by making use of this characteristic.
FIG. 6 shows a temperature sensing circuit 31 using a thermistor 29 attached to a cooling fin, a control circuit 26 for controlling an IGBT module 9a, and a gate drive circuit 12.
In the temperature sensing circuit 31, one end of the thermistor 29 is connected in series with a resistor 30 and a voltage source, and the junction between the thermistor 29 and the resistor 30 is connected to an input terminal of a voltage comparing circuit 13. The other end of the thermistor 29 is connected to ground potential. A second input terminal of the voltage comparing circuit 13 is connected to a constant voltage circuit 18, and the output terminal thereof is connected to the control circuit 26.
In FIG. 6, since the thermistor for temperature sensing is a part mounted on the cooling fin, a temperature change of the IGBT is sensed by the temperature of the cooling fin. Therefore, when an IGBT element 1a generates heat due to an over-current or the like, there arises a problem in that the temperature transmitted to the thermistor differs depending on the position of the element which is generating heat. In this regard, the temperature sensing accuracy is decreased as the distance between the element and the thermistor increases.